Toxicants applied to the exterior of rodents, referred to as contact toxicants, are of particular importance in those situations where conventional rodent devices, such as baits, are either difficult to apply or are poorly accepted.
Although contact toxicants can be directly applied (e.g. by spraying) to pest animals, the logistics of this approach make passive systems preferable. Passive contact toxicant systems are those wherein the pest animal encounters the toxicant during the course of it's normal movement or behavior. Most passive contact toxicant systems involve tracking powders which are ingested through subsequent grooming activity of the animal after exposure and body contamination. Such formulations, unfortunately, are unable to withstand changes in climatic conditions and therefore contaminate the environment.
Passive contact toxicants are further employed in control devices. An example of such a device is disclosed in U.S. Pat. No. 4,281,471. In this reference, an apparatus characterized by an elongated tube with an interior cross-section is disclosed. Within this interior cross-section is located at least one cartridge onto which is loaded an absorbent material commonly referred to as the "wick." The absorbent material is saturated with a toxic solution. Rodents passing through the elongated tube come into contact with the toxicant formulation. During grooming, the rodent ingests the adhered toxicant and subsequently dies, usually outside the area of contact. This apparatus offers maximum contact between the rodent and pesticide, while substantially reducing chances of contamination to non-target species.
Morris et al, "Design and Evaluation Criteria for Development of Toxic Wicks for Rodent Control", Special Technical Publication 817, pp. 165-182, 1984, discusses the feasibility of using similar devices in the control of rodents. In this publication, the toxicant composition applied to the wick is a 0.25% solution of brodifacoum in a propylene glycol solution containing polyethylene glycol of an approximate molecular weight of 200. Unfortunately, rodents often find such toxicant compositions attractive for nesting purposes and thus they readily remove them from the cartridges. Further, this toxic formulation readily absorbs water. Thus, the probability of the formulation leaking off the wick into the interior cross-section of the tube and, thus, into the environment is high. As a result, this composition may contaminate non-target species and the environment. Further, it is unsuitable for use in mass marketing due to it's susceptibility to physical changes in the atmosphere.